System and method for inventory management using mobile drive units

ABSTRACT

A method for inventory management includes deploying a first mobile drive unit having first dimensions and deploying a second mobile drive unit having second dimensions, the first and second dimensions being different. The first and second mobile drive units are operable to transport inventory items to a plurality of inventory stations in the same workspace.

TECHNICAL FIELD OF THE INVENTION

The invention relates in general to inventory management systems, andmore particularly to a system and method for inventory management usingmobile drive units.

BACKGROUND OF THE INVENTION

Over time, demands on modern inventory management systems haveincreased. Those demands include the need for more complex and diverseinventories, the need to sort and combine smaller quantities ofinventory in support of order fulfillment, and more efficient use oftime, space, equipment, and manpower. Accordingly, the inefficientutilization of system resources is increasingly unacceptable. Inventorymanagement systems unable to meet these increasing demands suffer fromlower system throughput, unfinished or delayed tasks, and unacceptableresponse times.

SUMMARY OF THE INVENTION

In accordance with the teachings of the present disclosure,disadvantages and problems associated with inventory management systemshave been substantially reduced or eliminated. In particular, aninventory management system is provided that includes mobile drive unitshaving different physical dimensions and capabilities.

In accordance with one embodiment of the present disclosure, a methodfor inventory management includes deploying a first mobile drive unithaving first dimensions and deploying a second mobile drive unit havingsecond dimensions, the first and second dimensions being different. Thefirst and second mobile drive units are operable to transport inventoryitems to a plurality of inventory stations in the same workspace.

In accordance with another embodiment of the present disclosure, asystem for inventory management includes a first mobile drive unithaving first dimensions and a second mobile drive unit having seconddimensions, the first and second dimensions being different. The systemalso includes a plurality of inventory holders. The first and secondmobile drive units are each operable to transport at least one of theinventory holders in the same workspace.

Technical advantages of certain embodiments of the present inventioninclude the ability to deploy a first mobile drive unit having firstdimensions and a second mobile drive unit having second dimensions in aninventory management system. In certain embodiments, the first andsecond mobile drive units may operate within the same workspace at thesame time. In some embodiments, the first and second mobile drive unitsmay have overlapping or distinct workspaces. Another technical advantagemay include transporting inventory holders that have different physicalsizes and/or purposes. Accordingly, in some embodiments different-sizedmobile drive units may each serve the same inventory station. Accordingto the teachings of the present disclosure, the space and time requiredto accomplish the same task as compared to a system that does not deploydifferent-sized mobile drive units may be substantially reduced and/orcertain operational efficiencies realized.

Other technical advantages of the present disclosure will be readilyapparent to one of ordinary skill in the art from the following figures,description, and claims. Moreover, while specific advantages have beenexplained above, various embodiments may include some, all, or none ofthose advantages.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present invention and itsfeatures and advantages, reference is now made to the followingdescription, taken in conjunction with the accompanying drawings, inwhich:

FIG. 1 is a block diagram illustrating an example embodiment of aninventory management system;

FIG. 2 is a perspective diagram illustrating an example embodiment of aninventory management system;

FIG. 3 is a block diagram illustrating an example embodiment of amanagement module according to an example embodiment of an inventorymanagement system;

FIGS. 4A and 4B are perspective diagrams illustrating techniques fordefining three-dimensional segment volumes with respect to an examplemobile drive unit and inventory holder in an example embodiment of aninventory management system;

FIG. 5 is a perspective diagram illustrating techniques for pathplanning and volume reservation according to an example embodiment of aninventory management system; and

FIGS. 6A and 6B are flowcharts illustrating an example embodiment of amethod for inventory management.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments of the present invention and its advantages are bestunderstood by referring to FIGS. 1 through 6, wherein like numeralsrefer to like and corresponding parts of the various drawings.

FIG. 1 is a block diagram illustrating an example embodiment of aninventory management system 10. Inventory management system 10 includesmanagement module 15, one or more mobile drive units 20, one or moremobile drive units 22, one or more inventory holders 30, one or moreinventory holders 32, and one or more inventory stations 50, 52, 54, and56. As illustrated, mobile drive units 20 may transport inventoryholders 30 within workspace 70, and mobile drive units 22 may transportinventory holders 32 within workspace 72. According to the teachings ofthe present disclosure, mobile drive units 20 and 22 may also operatewithin the same workspace 74 and may interface with the same inventorystations 54 and/or 56. Mobile drive units 20 may operate withinworkspaces 70 and 74, while mobile drive units 22 may operate withinworkspaces 72 and 74. Mobile drive units 20 may represent a type ofmobile drive unit of one dimension operable to transport inventoryholders 30 of certain dimensions, while mobile drive units 22 mayrepresent a different type of mobile drive unit of a second dimensionoperable to transport inventory holders 32 having different dimensionsthan inventory holders 30. As a result, inventory management system 10is capable of transporting inventory items between locations inworkspaces 70, 72, and/or 74 to facilitate the picking, replenishment,packing, and shipping of inventory items and other inventory managementtasks.

Management module 15 represents any suitable configuration of software,hardware, and controlling logic operable to facilitate and/or manage theoperations of inventory management system 10. Management module 15 mayinclude logic to schedule and execute various tasks involving themovement and processing of inventory items. Those tasks may includeassigning tasks to components of inventory management system 10, such asmobile drive units 20, mobile drive units 22, and/or workers orautomated equipment at inventory stations 50, 52, 54, and/or 56. Inaddition, management module 15 may, in some embodiments, manage andmaintain components of inventory management system 10. For example,management module 15 may provision portions of workspaces 70, 72, and/or74 for movement of mobile drive units 20 and/or 22, facilitate thedeployment of mobile drive units 20 and/or 22, schedule recharging ormaintenance of mobile drive units 20 and/or 22, manage storage ofinventory holders 30 and/or 32, and facilitate other appropriate tasks.Management module 15 may include one or more communication interfaces tosend information to and receive information from mobile drive units 20and/or 22, such as task assignments and movement coordination. In someembodiments, management module 15 may communicate with one or moreinventory stations 50, 52, 54, and/or 56. It should be noted that whilemanagement module 15 is depicted as a separate component withininventory management system 10, the functionality performed bymanagement module 15 may be distributed over several devices. Forexample, the operations of management module 15 may be performed bymobile drive units 20, mobile drive units 22, and/or other appropriatecomponents of inventory management system 10. As a result, any or all ofthe interactions between a particular mobile drive unit 20 andmanagement module 15 or mobile drive unit 22 and management module 15that are described below may, in particular embodiments, representpeer-to-peer communication between mobile drive units.

Mobile drive units 20 represent any suitable combination of devices andcomponents operable to move inventory holders 30 in workspace 70 and/orworkspace 74. Mobile drive units 20 may be designed to couple,interface, dock, or otherwise connect with inventory holders 30 in orderto move inventory items stored within inventory holders 30 to anyappropriate location within workspaces 70 and/or 74. For example, mobiledrive units 20 may be configured to move beneath a given inventoryholder 30 and lift inventory holder 30 using a docking head and/or otherappropriate coupling structure. Mobile drive units 20 may includeappropriate communications devices to communicate information to andfrom management module 15, other mobile drive units 20, mobile driveunits 22, and/or inventory stations 50, 52, 54, and/or 56. In someembodiments, mobile drive units 20 may communicate with managementmodule 15 and/or other mobile drive units using IEEE 802.11 (WiFi), IEEE802.15 (Bluetooth), infrared data association standards, or any otherappropriate wireless communication protocol. Mobile drive units 20 mayinclude appropriate sensors to detect fixed locations or other mobiledrive units 20 within workspaces 70 and/or 74 and thereby facilitatemovement and/or avoid collisions. Such information may be communicatedto management module 15 to assist in the function of inventorymanagement system 10. In some embodiments, mobile drive units 20 mayrepresent a type of mobile drive unit operable to transport pods and/orrack-shelf inventory holders 30.

Mobile drive units 22 represent any suitable combination of devices andcomponents to function in a similar manner to mobile drive units 20, butmay be of a different physical dimension suitable to move inventoryholders 32 in workspace 72 and/or workspace 74, where inventory holders32 have different dimensions than inventory holders 30. For example,mobile drive units 22 may have a different load carrying ability thanmobile drive units 20. In some embodiments, the physical size of amobile drive unit may correlate to its load carrying ability. Mobiledrive units 22 may thus represent a different type of mobile drive unitthan mobile drive unit 20. For example, mobile drive units 22 mayrepresent a type of mobile drive unit operable to transport palletsand/or case-sized inventory holders 32. It should be understood,however, that while specific examples of mobile drive unit 20 and mobiledrive unit 22 are discussed in the present disclosure, the teachings ofthe present disclosure are intended to allow many different types ofmobile drive units of varying physical dimensions and varyingcapabilities to operate within inventory management system 10.

Inventory holders 30 store inventory items 44. In some embodiments,inventory holders 30 include multiple storage bins, each capable ofholding one or more inventory items 44. Alternatively or in addition,inventory holders 30 may include a rack of shelves, each shelf capableof carrying one or more inventory items 44. Inventory holders 30 arecapable of coupling with mobile drive units 20 and being carried,rolled, or otherwise moved by mobile drive units 20. Inventory holder 30may include a plurality of faces. Each bin may be accessible through oneor more faces of the inventory holder 30. For example, in someembodiments, inventory holder 30 may include four faces. In someembodiments, inventory holders 30 may represent a type of inventoryholder suitable to hold orders, which may be assembled at inventorystations 50, 54, and/or 56. Mobile drive unit 20 may be operable torotate inventory 30 at appropriate times within workspaces 70 and/or 74to present a particular face containing the inventory items 44 needed atinventory stations 50, 54, and/or 56. Example embodiments of inventoryholders 30 are illustrated in greater detail in FIGS. 2, 4B, and 5.

Inventory holders 32 store inventory items 45. In some embodiments,inventory holders 32 may represent pallets and/or case-size inventoryholders. Inventory holders 32 may store bulk quantities of inventoryitems 45, larger inventory items 45, and/or assembled products 45.Inventory items 45 may, for example, represent bulk quantities of aninventory item 44. In some embodiments, inventory items 45 may be thesame as inventory items 44. In some embodiments, all of inventory items45 may have the same stock-keeping unit (SKU) and/or sometimes may havedifferent SKUs. Inventory holders 32 may be suitable for transportinginventory items 45 received into inventory management system 10 fromremote locations. Alternatively or in addition, inventory holders 32 mayrepresent a type of inventory holder suitable to hold orders, which maybe assembled at inventory stations 52, 54, and/or 56. For example,inventory holders 32 may transport outbound orders and/or shipments ofinventory items 45. Inventory holders 32 are capable of coupling withmobile drive units 22 and being carried, rolled, or otherwise moved bymobile drive units 22. Example embodiments of inventory holders 32 areillustrated in greater detail in FIGS. 2 and 5. It should be understood,however, that while embodiments of inventory holders 30 and/or 32 arediscussed in the present disclosure, the scope of the present disclosureis intended to encompass various inventory holders of any appropriatedimensions, including but not limited to inventory holders havingvarying heights, widths, lengths, and purposes. In addition, particularmobile drive units may be operable to transport more than one type ofinventory holder, and particular inventory holders may be operable to becarried by more than one type of mobile drive unit.

Inventory stations 50, 52, 54, and/or 56 represent locations designatedfor the completion of particular tasks involving inventory items. Suchtasks may include the removal of inventory items 44 and/or 45 frominventory holders 30 and/or 32, the introduction of inventory items 44and/or 45 onto inventory holders 30 and/or 32, and/or the counting ofinventory items 44 and/or 45 in inventory holders 30 and/or 32. In someembodiments, those tasks may include the decomposition of inventoryitems 45 from pallet or case-sized groups in inventory holders 32 to oneor more individual inventory items 44 into inventory holders 30, and/orthe composition of pallet or case-sized groups of inventory items 45 ininventory holders 32 from individual inventory items 44 in inventoryholders 30. Alternatively or in addition, tasks performed at inventorystations 50, 52, 54, and/or 56 may involve processing or handlinginventory items 44 and/or 45 in other suitable manners. For example,individual orders may be built and/or packed for shipment. As anotherexample, manufactured products may be built and/or assembled fromindividual parts 44. In some embodiments, inventory stations 50, 52, 54,and/or 56 may simply represent physical locations where a particulartask involving inventory items 44 and/or 45 can be completed withinworkspace 70, 72, and/or 74. In alternative embodiments, inventorystations 50, 52, 54, and/or 56 may represent both the physical locationand any appropriate equipment for processing or handling inventory items44 and/or 45, such as scanners for monitoring the flow of inventoryitems 44 and/or 45 in and out of inventory management system 10, andcommunication interfaces for communicating with management module 15.For example, inventory stations 50, 52, 54, and/or 56 may includedisplays for communicating tasks and other information to and from aworker or inventory manager. Inventory stations 50, 52, 54, and/or 56may be controlled by human operators or may be fully automated. Human orautomated operators in inventory stations 50, 52, 54, and/or 56 may becapable of performing certain inventory task items such as packing orcounting inventory items 44 and/or 45, as part of the operation ofinventory management system 10.

As depicted, inventory stations 50 may be capable of interfacing solelywith mobile drive units 20 within workspace 70. Thus, inventory stations50 may interact with inventory items 44 from mobile drive units 20.Likewise, inventory stations 52 may be capable of interfacing solelywith mobile drive units 22 within workspace 72. Thus, inventory stations52 may interact with inventory items 45 from mobile drive units 22.Inventory station 54, however, may represent a straddle transferinventory station in which one portion of inventory station 54interfaces with inventory holders 30 transported by mobile drive units20 and another portion of inventory station 54 interfaces with inventoryholders 32 transported by mobile drive units 22. Inventory stations 54may thus represent locations that support tasks involving both inventoryholders 30 and 32. Thus, inventory stations 54 may interact withinventory items 44 and 45 from mobile drive units 20 and 22. Forexample, mobile drive units 22 may transport pallet-sized and orcase-sized inventory holders 32 containing bulk inventory items 45 toinventory station 54 in order to split the inventory items 45 intoindividual inventory items 44 and move them to inventory holders 30.Inventory stations 56 represent inventory stations in which mobile driveunits 20 and 22 may interface universally within workspace 74. Thus,inventory stations 56 may interact with inventory items 44 and 45 frommobile drive units 20 and 22.

Workspace 70 represents an area associated with inventory managementsystem 10 in which mobile drive units 20 may move and where inventoryholders 30 can be stored. Likewise, workspace 72 represents an areaassociated with inventory management system 10 in which mobile driveunits 22 may move and where inventory holders 32 can be stored.Workspace 74 represents an area associated with inventory managementsystem 10 in which mobile drive units 20 and 22 may both move andrespectively transport inventory holders 30 and 32. In some embodiments,workspace 74 may represent an area associated with inventory managementsystem 10 in which workspaces 70 and 72 overlap. Although theillustrated figure shows an embodiment of inventory management system 10in which the overall workspace is divided into workspaces 70, 72, and74, particular embodiments of inventory management system 10 may includeworkspaces 70, 72, and 74 that have variable dimensions or an arbitrarygeometry. Accordingly, workspaces 70, 72, and 74 may be arranged inaccordance with any appropriate geometry suitable for the tasksassociated with inventory management system 10. For example, workspaces70, 72, and 74 may be arranged in accordance with the particulargeometries of a multi-level warehouse. As another example workspaces 70,72, and 74 may be arranged in accordance with receiving, picking, andshipping zones. Workspaces can apply to irregular settings such asairports, hospitals, and cargo ships.

As a result, while particular embodiments of inventory management system10 are illustrated, it should be appreciated that the teachings of thepresent disclosure may apply to any number and types of inventorymanagement systems. For example, inventory management system 10 mayrepresent a mail order warehouse, wherein individual orders may becompiled at inventory stations 50, 52, 54 and/or 56 for shipment toindividual customers. As another example, inventory management system 10may represent a merchandise return facility. In such embodiments,inventory items may represent merchandise returned by customers. Suchmerchandise may be stored in inventory holders 30 when received at thefacility from individual customers. At appropriate times, large numberof units may be removed from particular inventory holders 30 andtransferred at inventory station 54 and/or 56 to inventory holder 32 andpacked for shipment to a warehouse or other facility. As anotherexample, inventory management system 10 may represent an airport luggagefacility. Inventory items may represent pieces of luggage stored in theluggage facility. Mobile drive units 20 may retrieve inventory holders30 storing luggage arriving and/or departing on particular flights orluggage destined for particular types of processing such as x-ray ormanual searching. As yet another example, inventory management system 10may represent a manufacturing facility. In such embodiments, mobiledrive units 20 may be capable of facilitating a manufacturing process bytransporting inventory items 44 to be processed to the various inventorystations in inventory management system 10. For example, an inventoryitem 44 may be drilled, milled, and/or grinded at various inventorystations in inventory management system 10 in order to produce amanufactured product. Mobile drive units 22 may transport inventoryholders 32 holding case-sized and/or pallet-sized groups of unfinishedand/or finished goods. Although a number of example embodiments aredescribed in the present disclosure, inventory management system 10 may,in general, represent any suitable facility or system for storing orprocessing inventory items and inventory items may represent objects ofany type suitable for storage retrieval and/or processing in aparticular production or inventory management system 10.

FIG. 2 is a perspective diagram illustrating an example embodiment of aninventory management system 10. Within inventory management system 10,mobile drive units 20 may transport inventory holders 30 and mobiledrive units 22 may transport inventory holders 32 to various inventorystations 56 within the same workspace 74. Thus, each of mobile driveunits 20 and mobile drive units 22 may complete various tasks assignedby management module 15 within workspace 74.

To facilitate the function of inventory management system 10, managementmodule 15 may model and/or store a representation of workspace 74 as atwo-dimensional workspace. As illustrated, however, management module 15models and/or stores a representation of workspace 74 as athree-dimensional workspace. In addition or in the alternative,management module 15 may model and/or store a representation ofworkspace 74 in other coordination spaces as appropriate. Therepresentation of workspace 74 may include various fixed objects and/orbarriers within workspace 74 as well as the physical boundaries ofworkspace 74. In addition, management module 15 may store the locationsand corresponding representations of mobile drive units 20, mobile driveunits 22, inventory holders 30, and/or inventory holders 32 in twodimensions and/or three dimensions. Management module 15 may coordinateand/or assign tasks to the various components of inventory managementsystem 10. After assigning a task to a given mobile drive unit 20 and/or22, management module 15 may plan paths for mobile drive units 20 and/or22.

Management module 15 may enforce a reservation policy based on variousappropriate factors within workspace 74. For example, the reservationpolicy may be based on the location of a fixed object and/or thelocation of inventory holders 30 and/or 32. As another example, thereservation policy may be based on a predetermined number of maximumdrives for a given region of workspace 74, a predetermined maximumweight limit for the region, and/or other characteristics of mobiledrive units 20 and/or 22. In some embodiments, the reservation policymay be based on characteristics of inventory items 44 and/or 45, whichmay constrain certain inventory items 44 and/or 45 and/or types ofinventory items 44 and/or 45 to certain regions of workspace 74. Forexample, high value inventory and/or hazardous materials may beconstrained to certain regions of workspace 74. In addition or in thealternative, reservation policies may govern dynamic characteristics ofmovement of mobile drive units 20 and/or 22 within a region of workspace74. For example, a reservation policy may be based on maximum speedlimits, maximum acceleration/deceleration rates, and/or other dynamiccharacteristics of the movement. Reservation policies may also governvalid stopping locations within workspace 74 and/or may enforce minimumdistances between mobile drive units 20 and/or 22. Management module 15may determine whether particular reservation requests received frommobile drive units 20 and/or 22 may be granted based on the reservationpolicy and/or the locations and states of other mobile drive units 20and/or 22. In some embodiments, management module 15 may additionally oralternatively determine whether to apply restrictions to a grantedreservation based on the reservation policy. Management module 15 maydetermine whether reservation requests received from mobile drive units20 and/or mobile drive units 22 may be granted. More detailed techniquesfor implementing path planning and reservation policies will bediscussed below with respect to FIG. 5.

In an example embodiment of operation, mobile drive unit 20 is a type ofmobile drive unit operable to transport relatively smaller inventoryholders 30, while mobile drive unit 22 is a type of mobile drive unitoperable to transport relatively larger pallet or case-sized inventoryholders 32. Mobile drive units 20 and mobile drive units 22 are deployedwithin inventory management system 10. Management module 15 may thenfacilitate the administration and management of the various componentsof inventory management system 10.

Management module 15 may determine various tasks to complete orders.Management module 15 may, for example, determine that an order includingvarious inventory items 44 and/or 45 is to be assembled at inventorystations 56. Management module 15 determines the location of theinventory items 44 and/or 45 within the various inventory holders 30and/or 32 and assigns an appropriate number of mobile drive units 20and/or 22 to retrieve those inventory holders. Management module 15communicates tasks to mobile drive units 20 and/or 22 to retrieve thoseinventory holders 30 and/or 32 and transport them to inventory stations56. After receiving an appropriate task assignment from managementmodule 15, each of the mobile drive units 20 and/or 22 may move to itsassigned inventory holder 30 and/or 32, dock with inventory holder 30and/or 32, and transport inventory holder 30 and/or 32 to inventorystations 56. Once the inventory holders 30 and/or 32 are at theappropriate inventory station, a user may retrieve the appropriateinventory items 44 and/or 45 to complete the order. Once the order iscomplete, mobile drive units 20 and/or 22 may be assigned tasks totransport the inventory holders 30 and/or 32 to appropriate destinationswithin inventory management system 10, such as other inventory stations56 and/or storage locations. Once those tasks are complete, mobile driveunits 20 and/or 22 may indicate to management module 15 that they areavailable for additional task assignments.

Management module 15 may receive status information from variouscomponents of inventory system 10 and may respond appropriately. Forinstance, management module 15 may receive information indicating wheneach of the tasks described above are completed. Management module 15may assign additional tasks accordingly. As another example, managementmodule 15 may receive information indicating that inventory items 45have been received into inventory management system 10 from inventorystations 52. In response, management module 15 may transmit tasks tomobile drive units 22 to transport those inventory items 45 in inventoryholders 32 to appropriate destinations within workspace 72 and/or 74.For instance, management module 15 may assign a task to a given mobiledrive unit 22 to move an inventory holder 32 to a storage location inworkspace 72 and/or 74 for storage. As another example, managementmodule 15 may assign a task for a given mobile drive unit 22 totransport an inventory holder 32 to inventory station 56 so thatinventory items 44 may be disassembled and stored in inventory holders30. Accordingly, management module 15 may assign tasks to mobile driveunits 20 to transport inventory holders 30 having available storagespace to inventory station 56. A user may then disassemble inventoryitems 45 into individual inventory items 44 and store those inventoryitems 44 into inventory holders 30. While the above operation isexplained by way of example, it should be understood that any number andtype of tasks may be completed at inventory stations 56 involvinginventory holders 30 and/or 32 and mobile drive units 20 and/or 22. Forexample, inventory items 44 from inventory holders 30 may be used tobuild pallet or case-sized inventory holders 32 of inventory items 45.

Management module 15 may also assign inventory tasks based on thecapabilities of inventory stations 50, 52, 54, and 56 and/or inventoryrestrictions associated with those inventory stations. For example,management module 15 may store and/or determine the ability forinventory items to be transported to given inventory stations 50, 52,54, and/or 56. Based on the inventory items 44 and/or 45 accessible to agiven inventory station 50, 52, 54, and/or 56, management module 15 mayassign tasks to that inventory station 50, 52, 54, and/or 56 associatedwith those inventory items 44 and/or 45. Management module 15 may alsoassign certain inventory tasks to inventory stations 50, 52, 54, and/or56 based on the capabilities of those inventory stations 50, 52, 54,and/or 56 to perform certain tasks. For example, certain inventorystations 52, 54, and/or 56 may be capable of processing pallets 32. Whenprocessing a given order, management module 15 may determine that aninventory item 45 appropriate for the order is in pallet 32. Based onthat determination, management module 15 may assign that order to aninventory station 52, 54, and/or 56 capable of processing pallets 32.When processing the order, management module 15 may also determine, forexample, that an inventory item 44 appropriate for the order is in agiven inventory holder 30. Management module 15 may then assign theorder to an inventory station 54 and/or 56 capable of processinginventory items 44 and pallets 32. Management module 15 may additionallyor alternatively assign orders to inventory stations 50, 52, 54, and/or56 based on the locality of inventory holders 30 and/or 32 appropriatefor those orders within inventory management system 10.

FIG. 3 is a block diagram illustrating an example embodiment ofmanagement module 15 according to an example embodiment of an inventorymanagement system 10. As illustrated, management module 15 includesprocessor 100, memory 102, communication interface 104, resourcescheduling module 106, route planning module 108, and space reservationmodule 110.

Processor 100 represents any suitable collection of hardware, software,and/or controlling logic operable to control the operation andadministration of elements within management module 15. For example,processor 100 may operate to process information and/or commandsreceived from communication interface 104, memory 102, resourcescheduling module 106, route planning module 108, and/or spacereservation module 110. Processor 100 may be a microcontroller,processor, programmable logic device, application-specific integratedcircuit, field programmable gate array, digital signal processor, and/orany other suitable specific or general purpose processors. In someembodiments, processor 100 may represent multiple parallel and/ormulti-core processors.

Memory 102 represents any one or a combination of volatile ornon-volatile, local or remote devices suitable for storing information.For example, memory 102 may include random access memory, read-onlymemory, magnetic storage devices, optical storage devices, hard disks,flash memory, or other suitable information storage device orcombination thereof. Accordingly, memory 102 stores either permanentlyor temporarily files and other information such as resource module 106,route planning module 108, space reservation module 110, and/or anyother appropriate code or instructions operable to facilitate theoperation of management module 15. In some embodiments, memory 102 maystore, either temporarily or permanently, serial input/output receivedfrom communication interface 104.

Communication interface 104 represents any suitable collection ofhardware, software, and/or controlling logic capable of communicatinginformation to and receiving information from various elements withininventory management system 10, such as mobile drive units 20, mobiledrive units 22 and/or inventory stations 50, 52, 54, and/or 56.Communication interface 104 may represent a network interface card,Ethernet card, port application-specific integrated circuit, wirelesscommunication module, or other appropriate interface operable tocommunicate with elements of inventory management system 10. Forexample, management module 15 may communicate with inventory stations50, 52, 54, 56, mobile drive units 20, and/or mobile drive units 22using IEEE 802.11 (WiFi), IEEE 802.15 (Bluetooth), infrared dataassociation standards, or any other appropriate wireless communicationprotocol.

In general, resource scheduling module 106, route planning module 108,and space reservation module 110 represent any appropriate combinationof hardware, software, controlling logic, algorithms, or heuristics,operable to provide the functionality described in the presentdisclosure. Example functionality of each module is described in moredetail below. It should be understood, however, that while described asseparate modules, the functionality provided by each of resourcescheduling module 106, route planning module 108, and space reservationmodule 110 may be provided by one module or many modules and/or may bedistributed among various components of inventory management system 10.

Resource scheduling module 106 processes received inventory requests andgenerates one or more assigned inventory tasks to be completed by thevarious components of inventory management system 10. Resourcescheduling module 106 may also select one or more appropriate componentsfor completing the assigned tasks and, using communication interface104, may communicate the assigned tasks to the relevant components.Resource scheduling module 106 may utilize the current assignment stateof various components within inventory management system 10 tofacilitate the assignment of tasks. Additionally, resource schedulingmodule 106 may be responsible for generating assigned miscellaneoustasks associated with various management operations such as assigningtasks to mobile drive units 20 and mobile drive units 22. For example,resource scheduling module 106 may request mobile drive units 20 and/or22 to recharge batteries or have batteries replaced, instruct inactivemobile drive units 20 and/or 22 to park in an appropriate location,and/or direct mobile drive units 20 and/or 22 to appropriate locationsfor repair and/or scheduled maintenance. In some embodiments, resourcescheduling module 106 may assign tasks to various inventory stations 50to indicate orders for packing, inventory items for unpacking, and/orother appropriate tasks along with appropriate information necessary tocomplete such tasks. For example, resource scheduling module 106 maycommunicate one or more tasks to be completed by workers located atinventory stations 50. Additionally or alternatively, resourcescheduling module 106 may receive status updates from the variouscomponents of inventory management system 10. Accordingly, resourcescheduling module 106 may update its internal resource scheduleappropriately.

Route planning module 108 receives route requests from mobile driveunits 20 and/or 22. Route requests may identify one or more destinationsassociated with the tasks assigned by resource scheduling module 106.Route requests may also identify the location of the mobile drive unit20 and/or 22 making the request. In response to receiving a routerequest, route planning module 108 determines a path from the locationof the requesting mobile drive unit 20 and/or 22 to one or moredestinations identified in the route request. Route planning module 108may implement any appropriate algorithms utilizing any appropriateparameters, factors and/or considerations to determine an appropriatepath within workspace 74. For example, route planning module 108 mayutilize a three-dimensional representation of workspace 74 to determinethe locations of fixed objects and/or other components of inventorymanagement system 10. Route planning module 108 may then plan a routeappropriately based on the locations of those objects and/or components.After determining an appropriate path, route planning module 108 mayutilize communication interface 104 to transmit a route responseidentifying the path to the requesting mobile drive unit 20 and/or 22.In some embodiments, route planning module 108 may determine multiplepaths for each mobile drive unit 20 and/or 22.

Space reservation module 110 receives reservation requests from mobiledrive units 20 and/or 22 that are attempting to move along pathsdetermined by route planning module 108. Each path may be divided into aseries of segments. Each reservation request may attempt to reserve oneor more segments along the determined path. For each segment of thedetermined path, a requesting mobile drive unit 20 and/or 22 attempts toreserve the amount of space required to move along that segment. In someembodiments, the amount of space requested corresponds to thetwo-dimensional footprint of the requesting mobile drive unit 20 and/or22, along with the footprint of any associated load that the mobiledrive unit 20 and/or 22 may be carrying, such as inventory holder 30and/or 32. In other embodiments, the amount of space requestedcorresponds to the amount of three-dimensional volume required to movealong that segment. In particular, a requesting mobile drive unit 20and/or 22 may attempt to reserve one or more sets of workspace volumesassociated with the physical dimensions of mobile drive unit 20 and/or22, along with any load the mobile drive unit 20 and/or 22 may becarrying. Likewise, for each segment of the determined path, arequesting mobile drive unit 22 attempts to reserve one or more sets ofworkspace volumes associated with the physical dimensions of mobiledrive unit 22. Space reservation module 110 may determine a number ofworkspace volumes required for a given mobile drive unit based on asegment volume associated with that mobile drive unit. These techniquesare explained in more detail below with respect to FIGS. 4A, 4B, and 5.

Space reservation module 110 determines whether the requesting mobiledrive unit 20 and/or 22 is allowed to use the requested space based onvarious factors. For example, space reservation module 110 may determinewhether the requested set of three-dimensional workspace volumesintersects with any other workspace volumes reserved by anothercomponent of inventory management system 10, such as a fixed object oranother mobile drive unit 20 and/or 22. Thus, space reservation module110 may ensure that no physical interferences take place withinworkspace 74. Space reservation module 110 may then determine whetherthe requested space conflicts with a reservation policy. In someinstances, a reservation policy may be overridden based on the nature ofthe reservation policy. After determining whether the reservationrequest may be granted, space reservation module 110 transmits, usingcommunication interface 104, a reservation response granting or denyingthe reservation request to the requesting mobile drive unit 20 and/or22.

FIGS. 4A and 4B are perspective diagrams illustrating techniques fordefining three-dimensional segment volumes with respect to an examplemobile drive unit 20 and inventory holder 30 in an example embodiment ofan inventory management system 10. In certain embodiments, mobile driveunits 20 and/or 22 and inventory holders 30 and/or 32 may be modeled bythe use of three-dimensional shapes, or volume segments, to facilitatemovement within workspace 74. Accordingly, each mobile drive unit 20 andmobile drive unit 22 are resolved into a volume segment that representsthe space required by the respective mobile drive unit to move withinworkspace 74. The volume segment for each mobile drive unit may be usedby management module 15 to determine which workspace volumes withinworkspace 74 are requested by each reservation request.

As illustrated in FIG. 4A, the three-dimensional volume segment 200 formobile drive unit 20 is represented by the intersection between arectangular cube and a cylindrical shape. Volume segment 200 mayrepresent the amount of space required by the mobile drive unit 20 tomove within a given space in workspace 74. In some embodiments, volumesegment 200 may be projected along the length of a given segment 17 toform volume segment 204. Thus, when mobile drive unit 20 transmits areservation request for segment 17, the set of workspace volumesrequested may correspond to volume segment 204. Additionally oralternatively, volume segment 200 may be modified to represent thevolume required by mobile drive unit 20 to rotate within a given space.It should be noted that the illustrated volume segment is provided as anexample corresponding to a particular embodiment of mobile drive unit20, and a similar technique may be employed to define volume segmentsfor any number and type of mobile drive units, including mobile driveunits 22.

As illustrated in FIG. 4B, an example inventory holder 30 is representedas a volume segment 300 including a rectangular cube for the main bodyof inventory holder 30 and four rectangular cubes representing the legsof inventory holder 30. As illustrated, a volume segment 200representing mobile drive unit 20 may be modified to include volumesegment 300 when mobile drive unit 20 docks with inventory holder 30. Insome embodiments, volume segment 200 and volume segment 300 may beprojected along the length of a given segment 17 to form volume segment304. Thus, when mobile drive unit 20 transmits a reservation request forsegment 17, the set of workspace volumes requested may correspond tovolume segment 304. A similar technique may be employed with respect tomobile drive units 22 and inventory holders 32.

Accordingly, mobile drive units 20, mobile drive units 22, inventoryholders 30, inventory holders 32, and various fixed objects withinworkspace 74 may be modeled as three-dimensional volumes. Based on thesevolumes, a reservation policy may be applied to facilitate planningpaths for mobile drive unit 20 and mobile drive unit 22 throughoutworkspace 74. More details on such techniques will be illustrated withrespect to FIG. 5 below.

It should be noted, however, that similar modeling techniques may beemployed to represent mobile drive units 20, 22, and any associatedloads in two-dimensional space. For example, in FIG. 4A, footprint 306may represent mobile drive unit 20 in two-dimensional space, while inFIG. 4B, footprint 308 may represent mobile drive unit 20 and inventoryholder 30 in two-dimensional space. Techniques for coordinating themovement of mobile drive units 20 and 22 in workspace 74 usingtwo-dimensional representations are also explained below.

FIG. 5 is a perspective diagram illustrating techniques for pathplanning and space reservation according to an example embodiment ofinventory management system 10. In the illustrated examples, mobiledrive units 20 and/or 22 receive various tasks from resource schedulingmodule 106 of management module 15. Based on the respective assignedtasks, mobile drive units 20 and/or 22 may request, from route planningmodule 108 of management module 15, a path to a destination associatedwith their respective assigned tasks. For example, as illustrated,mobile drive unit 20 a may receive a task to retrieve inventory holder30 a from a storage location associated with inventory holder 30 a inworkspace 74. Mobile drive unit 20 a may then request a path to thestorage location. Likewise, mobile drive unit 20 b may receive a task totransport an inventory holder 30 b from inventory station 56 to adestination within workspace 74 for storage and may then request a pathto the storage location. Meanwhile, mobile drive unit 22 a may receive atask to transport a pallet inventory holder 32 a from inventory station52 to inventory station 56 and may then request a path to inventorystation 56.

In the illustrated embodiment, workspace 74 may be associated with agrid of three-dimensional workspace volumes 75. Workspace volumes 75within the three-dimensional space may be indexed according to a globalcoordinate system having x, y, and z coordinates. In some embodiments,workspace 74 may additionally or alternatively be associated with atwo-dimensional floor layout and/or map of workspace 74. In someembodiments, workspace 74 may be represented in two-dimensional space bya grid of workspace cells 76. In other embodiments, however, workspace74 may be represented in two-dimensions without the use of workspacecells 74. For example, workspace 74 may be represented bytwo-dimensional footprints and/or projections of the various objectswithin inventory management system 10. Thus, a given location inworkspace 74 may be identified by a set of workspace volumes 75, one ormore workspace cells 76, and/or by the location of its two-dimensionalfootprint. It should be understood, however, that while certainembodiments of inventory management system 10 may include only workspacevolumes 75, only workspace cells 76, or only the two-dimensionalfootprints of physical and/or virtual objects, other embodiments mayinclude any combination of workspace volumes 75, workspace cells 76,and/or two-dimensional footprints.

Workspace volumes 75 and/or workspace cells 76 describing workspace 74may be described in any appropriate increments. For example, in someembodiments, workspace volumes 75 may be described in millimeterincrements. Workspace cells 76, in some embodiments, may be described aslarger increments than workspace volumes 75. For example, workspacecells 76 may be described in increments of fifty or more centimetersand/or as appropriate based on the physical dimensions of mobile driveunits 20, mobile drive units 22, inventory holders 30, and/or inventoryholders 32. In some embodiments, workspace 74 may be associated with twosets of workspace cells 76, with one set described in incrementscorresponding to mobile drive units 20 and a second set described inincrements corresponding to mobile drive units 22. Mobile drive units 20and/or 22 may be configured to move within workspace 74 by navigatingfrom the center of one workspace cell to the center of another workspacecell. Additionally or in the alternative, workspace 74 may be defined asa set of nodes, which may indicate valid stopping points, turningpoints, junctions, and/or intersections. Nodes may be indicated with theuse of fixed object markers, or fiducials. When a node is detected,mobile drive units 20 and/or 22 may determine that a given destinationassociated with that node has been reached. Nodes may be connected byedges. In such embodiments, route planning module 108 may plan pathsfrom node to node, evaluating space needed to pass from one node to thenext to determine viability. It should be noted, however, that while anode may indicate a valid stopping point, that does not mean a mobiledrive unit 20 and/or 22 is required to always stop and/or turn at thatnode. Likewise, mobile drive units 20 and/or 22 may stop and/or turn atother locations within workspace 74 besides at nodes.

Based on one or more characteristics of a given space within workspace74, an appropriate component of management module 15 may assign and/ordetermine a state of that space. For example, management module 15 mayassign a state to a given workspace volume 75, workspace cells 76,and/or two-dimensional footprints of objects within inventory managementsystem 10. Example states may include occupied, reserved, and/or empty.In some embodiments, the state of a given workspace volume 75, workspacecell 76, and/or footprint may include identifying information associatedwith the component occupying that workspace volume 75, workspace cell76, and/or footprint. For example, management module 15 may determinethat an inventory holder 30 and/or 32 is being stored within one or moreworkspace cells 75 and/or workspace volumes 76, and the state isrepresented as occupied and/or reserved. Alternatively or in addition,management module 15 may determine that a given footprint is atwo-dimensional projection of an inventory holder 30 and/or 32 onto therepresentation of workspace 74. In some embodiments, inventorymanagement system 10 may individually identify particular componentssuch as mobile drive units or inventory holders. Accordingly, a givenobject within workspace 74 may be represented in three dimensions byassigning a state associated with that object to a set of workspacevolumes 75 associated with the total volume of that object. Managementmodule 15 may store the state of each of the sets of workspace cells 76,workspace volumes 75, and/or footprints in memory 102 in order tofacilitate the assignment of tasks and/or movements of mobile driveunits 20 and/or 22.

Management module 15 may associate a reservation status with variousspaces within inventory management system 10. For example, managementmodule 15 may associate a reservation status with workspace volumes 75,workspace cells 76, and/or footprints of objects within inventorymanagement system 10. Reservations may have types according to areservation policy. Reservation policies may govern whether reservationrequests may be granted. Reservation policies may be based on variouspath planning and/or path reservation constraints. In embodiments whereworkspace 74 is modeled in three dimensions, for example, reservationpolicies may determine whether a path may be planned through a given setof volumes and/or whether a given set of volumes along that path may bereserved. Path planning constraints may include geographic limitationsof workspace 74 such as fixed objects, walls, and other physical and/orvirtual barriers. In some embodiments path planning constraints mayinclude idle mobile drive units and/or stored inventory holders. Pathreservation constraints may include inventory holder locations, thelocations of other moving mobile drive units, and maximum drives allowedin a given region. Volumes representing any number of fixed objects 62and/or other physical components of workspace 74 may hold a reservationon a given set of workspace volumes 75. In some embodiments, areservation may be held by a virtual object and/or barrier such as adoorway or other logical barrier. Similar techniques may be used fortwo-dimensional representations of workspace 74.

A reservation may be permanent, temporary, or conditional. Accordingly,a fixed object 62 may have a permanent reservation policy applied to itand thus ensure that route planning module 108 cannot plan a path formobile drive units 20 and/or 22 through an area of workspace 74containing a fixed object 62. In some embodiments, idle mobile driveunits and/or stored inventory holders may have a permanent or temporaryreservation policy applied to them. Mobile drive units 20, mobile driveunits 22, inventory holders 30, and/or inventory holders 32, however,may have temporary reservation policies applied to them. Some portionsof workspace 74, such as workspace region 78 surrounding inventorystation 56, may have a conditional reservation policy applied to it.Accordingly, in some embodiments, management module 15 may conditionallygrant reservations in workspace region 78 only to mobile drive unitshaving certain characteristics. Additionally or alternatively,management module 15 may limit the number and/or types of mobile driveunits allowed within workspace region 78. For example, management module15 may set a maximum number of allowed mobile drive units withinworkspace region 78 and/or may only allow mobile drive units 20 atcertain times and mobile drive units 22 at other times within workspaceregion 78. In some embodiments, workspace region 78 may allow bothmobile drive units 20 and mobile drive units 22 at the same time.Additionally or alternatively, workspace region 78 may allow mobiledrive unit reservations based on the owner or administrator of themobile drive unit. Thus, a set of reservation policies may be associatedwith workspace 74 to facilitate the reservation of workspace volumes 75within inventory management system 10. In embodiments where workspace 74is represented in two dimensional space, reservation policies describingheight restrictions may be applied to given locations within workspace74. Thus, in such embodiments, while a reservation request may notdetermine that the requested space intersects with a reserved volume, areservation policy may restrict the movement of mobile drive units 20and/or 22 that may be too tall to traverse through a given area.

Upon receiving a path request from mobile drive units 20 and/or 22,route planning module 108 of management module 15 may determine a pathto the destination based on the current location of the respectivemobile drive unit and/or other appropriate characteristics of workspace74. For example, route planning module 108 may determine a node-to-nodepath from the current location of the mobile drive unit 20 and/or 22 toits chosen destination. In some embodiments, route planning module 108may determine an appropriate path based on the current state of each ofthe workspace cells 76, workspace volumes 75, or footprint of objectswithin workspace 74. For example, route planning module 108 may plan thepath based on the reservation status of workspace volumes 75. In someembodiments, route planning module 108 may determine the path based onthe state of workspace volumes 75, workspace cells 76, and/or footprintscorresponding to the type of mobile drive unit requesting the path.

Route planning module 108 may plan the path based on existing volumetricreservation data. In some embodiments, route planning module 108 mayplan the path based on one or more reservation policies associated withone or more workspace volumes 75 in workspace 74. In some embodiments,those reservation policies are based on path planning constraints. Forexample, route planning module 108 may plan to avoid any permanentvolume reservations held by fixed objects 62 such as poles, stairways,overhead obstacles, walls or other fixed barriers. In some embodiments,route planning module 108 may also plan to avoid idle mobile drive unitsand/or stored inventory holders. Temporary and/or conditionalreservations may be released and/or satisfied before a given componentattempts to reserve that space. Route planning module 108 mayaccordingly include temporary and/or conditional reservations in theplanned paths. For example, a set of workspace volumes 75 temporarilyreserved by a mobile drive unit 20 a may be included in a planned pathfor mobile drive unit 20 b.

As discussed above, route planning module 108 may take into account thephysical volume of a given mobile drive unit when planning paths. Insome embodiments, route planning module 108 may determine that thevolume segment representing the physical volume of mobile drive unit 20and/or 22 will intersect a reserved space within workspace 74.Alternatively or in addition, a reservation policy may govern any heightrestrictions associated with a given area of workspace 74. Thus, basedon volume intersections and/or height restriction policies, routeplanning module 108 may plan paths for mobile drive units 20 that wouldnot be possible or acceptable if taken by mobile drive units 22. Forexample, route planning module 108 may plan a path for mobile drive unit20 to pass below an overhead obstacle that would be impassible by mobiledrive unit 22 due to mobile drive unit 22's greater height. Likewise,route planning module 108 may determine not to plan the same pathbeneath the overhead obstacle if mobile drive unit 20 is transportinginventory holder 30.

Route planning module 108 may then communicate information identifyingthe path to the requesting mobile drive units 20 and/or 22. For example,as illustrated, route planning module 108 transmits path 16 a to mobiledrive unit 20 a, path 16 b to mobile drive unit 20 b, and path 16 c tomobile drive unit 22 a. A given path may be divided into one or moresegments 17, which may specify intermediate points along that path. Insome embodiments, a segment may correspond to the length from the centerof one workspace cell to the center of another workspace cell.Additionally or alternatively, route planning module 108 may determinemultiple alternative paths to a given destination. Route planning module108 may communicate information to mobile drive units 20 and/or 22identifying alternative paths and/or may prioritize paths based onvarious appropriate cost factors such as time, length, and/orefficiency.

Once route planning module 108 has transmitted paths 16, the informationis received by each respective mobile drive unit 20 and/or 22. Once amobile drive unit 20 and/or 22 receives path 16, the respective mobiledrive unit then attempts to reserve segment 17, which may comprise oneor more workspace volumes 75 associated with the dimensions of mobiledrive unit 20 and/or 22, as discussed above with respect to FIGS. 4A and4B. Alternatively, segment 17 may represent a two-dimensional footprintassociated with the dimensions of mobile drive unit 20 and/or 22. Inother embodiments, segment 17 may represent one or more workspace cells76. In some embodiments, mobile drive units 20 and/or 22 may transmitinformation including a reservation request to space reservation module110 of management module 15. For example, volume segment 200 mayrepresent mobile drive unit 20 a. Mobile drive unit 20 a may request toreserve segment 17 a of path 16 a, where segment 17 a comprises volumesegment 204, where volume segment 204 is a projection of volume 200along path 16 a. As another example, volume segment 300 may representmobile drive unit 20 b and its associated load, which is inventoryholder 30 b in the illustrated example. Mobile drive unit 20 b mayrequest a segment of path 16 b comprising volume segment 304, wherevolume segment 304 is a projection of volume 300 along path 16 b. Asanother example, volume segment 400 may represent mobile drive unit 22 aand its associated load, which is inventory holder 32 a and inventoryitem 45 in the illustrated example. Mobile drive unit 22 a may request asegment of path 16 c comprising volume segment 404, where volume segment404 is a projection of volume 400 along path 16 c.

Once space reservation module 110 receives a reservation request for asegment 17 of path 16, space reservation module 110 determines whetherto grant the space representing the requested segment. For example, therequested space may be a set of one or more workspace volumes 75representing the requested volume segment 17. In some embodiments, spacereservation module 110 determines whether the requested segment 17intersects with space that has been temporarily and/or conditionallyreserved by a component of inventory management system 10. For example,an inventory holder 30 may hold a temporary reservation on a set ofworkspace volumes 75 corresponding to volume 300. In some embodiments, asmaller portion of the requested set of workspace volumes 75 may begranted. Additionally or alternatively, portions of the originallyrequested set of workspace volumes 75 may be granted as theirreservation status changes from reserved by a different object tounreserved. For example, two mobile drive units 20 and/or 22 may requestsegments 17 along the same region of workspace 74, such as an aislebetween rows of inventory holders 30 and/or 32. The leading mobile driveunit 20 or 22 may be given some or all of the workspace volumes 75 thatit requests. As that mobile drive unit 20 or 22 progresses alongsegments 17, the mobile drive unit 20 or 22 behind it may be given newportions of the originally requested segment 17 as those portions becomeavailable due to the evacuation of the leading mobile drive unit 20 or22. Accordingly, one mobile drive unit 20 or 22 may follow anothermobile drive unit 20 or 22 when they are moving in the same direction.

Space reservation module 110 determines whether to grant the requestedsegment based on determining whether the requested segment intersectswith temporarily reserved space, such as space reserved by one or moreworkspace volumes 75, workspace cells 76, and/or footprints of objectsin inventory management system 10. Space reservation module 110 may alsodetermine whether the requested segment conflicts with a conditionalreservation policy. For example, a requested volume segment may conflictwith a conditional reservation policy associated with the requestedworkspace volumes 75. If the space reservation module 110 determinesthat the request can be granted, a reservation response will betransmitted to the requesting mobile drive unit 20 and/or 22 and themobile drive unit 20 and/or 22 will then traverse the requested segment.If the request is not granted, the requesting mobile drive unit may thenrequest the segment again, may request a new path, and/or may request asegment from an alternate path according to a priority set by routeplanning module 108. Once mobile drive unit 20 has successfully reservedsegment 17, mobile drive unit 20 moves along segment 17. In someembodiments, if a given mobile drive unit, such as first mobile driveunit 20 fails, then a space, such as a set of workspace volumes 75, maybe reserved large enough to account for any positional uncertainty ofthe mobile drive unit. At some point, after beginning movement, mobiledrive unit 20 may request the next segment. For example, it may requesta segment, before, during or after the traversal along segment 17 a hasbeen requested. The process of requesting volume segments 204 isrepeated along path 16 a until the desired destination is reached. Forexample, as illustrated, mobile drive unit 20 a requests segment 17 a tosegment 17 h corresponding to path 16 a, each segment 17 comprisingvolume segments 204. It should be noted that as illustrated, path 16 aincludes one or more turns and/or curves. At such turns, mobile driveunit 20 a may request a rotation reservation, in which mobile drive unit20 a requests workspace volumes 75 corresponding to the amount of space206 required to execute a rotation maneuver. Mobile drive unit 20 a maycontinue reserving segments 17 of path 16 a until reaching thedestination. Thus, mobile drive unit 20 a may move to inventory holder30 a in workspace 74. Mobile drive units 20 b and 22 a may traversetheir respective paths 16 b and 16 c in like manner by requesting toreserve volume segments associated with their respective dimensions.

Space reservation module 110 may determine that the requested segment isphysically occupied by an object such as inventory holder 30 and/orinventory holder 32, or other mobile drive units 20 and/or 22. Spacereservation module 110 may then determine that the requested portion ofthe requested segment is not reserved by the three-dimensional volumerepresenting the physical object. For example, there may be a spacebeneath inventory holder 30 or 32 under which mobile drive unit 20and/or 22 may pass. Thus, by reserving the unreserved portion beneathinventory holder 30 or 32, mobile drive units 20 and/or 22 may movebeneath inventory holder 30 or 32. In some embodiments, such techniquesmay be employed by route planning module 108 to plan paths for mobiledrive units 20 and/or 22. A similar technique may be used such thatmobile drive units may move beneath inventory holders in order to coupleand/or dock with them. Additionally or alternatively, height restrictionpolicies and/or tunneling policies associated with particular objectssuch as inventory holders 30 and/or 32 may enable similar techniques inembodiments where workspace 74 is represented in two-dimensional space.

Space reservation module 110 may determine that the requested segment ispartially reserved and partially unreserved. For example, spacereservation module 110 may determine that the unreserved portioncomprises a part of segment 17 that another mobile drive unit 20 and/or22 has recently exited. Space reservation module 110 may partially grantthe request of the unreserved portion of the requested segment 17. Insome embodiments, mobile drive units 20 and/or 22 may releasereservations on a volume segment 17 once those mobile drive units 20and/or 22 have moved through that space. Space reservation module 110may grant the additionally released portions of segment 17 as theybecome available. In some embodiments, however, the entirety of theadditionally released portions may not be granted. Space reservationmodule 110 may determine that granting the additional portion wouldconflict with a reservation policy. For example, granting the additionalportion may conflict with a policy that enforces valid stopping pointsat fiducial markers and/or ensures minimum distances between mobiledrive units. Based on that determination, space reservation module 110may reduce the additional portion so that the reservation policy may bemaintained. Alternatively or additionally, space reservation module 110may determine the direction of movement of the mobile drive unit thatholds the reservation on a requested segment and if the direction ofmovement of that mobile drive unit is in the same direction as therequesting mobile drive unit, then space reservation module 110 maygrant that request. Using such techniques, mobile drive units 20 and/or22 may move more closely behind other mobile drive units than wouldotherwise be possible.

In the illustrated example, once mobile drive unit 20 a reachesinventory holder 30 a, mobile drive unit 20 a may move beneath inventoryholder 30 a and may then couple with inventory holder 30 a. Oncecoupled, mobile drive unit 20 a may transmit information identifyingthat it has coupled with inventory holder 30 a. Space reservation module110 may thus determine that mobile drive unit 20 a has coupled withinventory holder 30 a. Accordingly, space reservation module 110 maymodify the volume segment 200 representing mobile drive unit 20 a basedon volume segment 300, which represents the physical dimensions ofinventory holder 30 a. Thus, when mobile drive unit 20 a attempts tomove or transport inventory holder 30 a to another destination, such asinventory station 56, the reservation request will be based on themodified volume segment 304 representing volume segment 200 of mobiledrive unit 20 and volume segment 300 of inventory holder 30 a.

In some embodiments, mobile drive units 20 and/or 22 may request varioustypes of reservations based on different types of movement. For example,while the illustrated examples demonstrate straight line movements,mobile drive units 20 and/or 22 may be operable to request spacesufficient to allow for mobile drive units 20 and/or 22 to rotateinventory holders 30 and/or 32. Accordingly, mobile drive units 20and/or 22 may be configured to request to reserve a segment 17 thatincludes space adjacent to the area through which path 16 runs. Forexample, segment 17 may be defined by a volume resulting from rotating aprojection of the volume segment representing the physical dimensions ofmobile drive units 20 and/or 22 about an axis of rotation. Similarly,segment 17 may be defined by the two-dimensional footprint resultingfrom that rotation. Consequently, when a requesting mobile drive unit 20and/or 22 plans to rotate inventory holder 30 and/or 32 as part of itsmovement in completing a particular segment 17, the requesting mobiledrive unit 20 and/or 22 may attempt to place rotation reservations onthe space adjacent to the space in which the mobile drive unit intendsto perform the rotation, which is the space corresponding to the spacerequired to rotate the mobile drive unit 20 and/or 22. Accordingly,mobile drive units 20 and/or 22 need only reserve enough space necessaryto perform the rotation maneuver, although more space may be reserved asappropriate. Because mobile drive units 20 and/or 22 may only reserveenough space within workspace 75 in which to perform the rotationmaneuver, more space is available for other inventory management tasks.This may reduce the number of delays mobile drive units face whenattempting to reserve a sufficiently large portion of workspace 74 torotate inventory holders 30 and/or 32. In such embodiments, a requestwould be granted based on the determination that the space requested forthe rotation maneuver does not intersect with a space reserved byanother mobile drive unit or fixed object.

Alternatively or in addition, reservation requests may be made based onworkspace cells 76 in a like manner to that which is described above.Accordingly, depending upon the size of inventory holders 30 and/or 32relative to workspace cells 76 utilized in the relevant workspace 74,the requesting mobile drive unit 20 and/or 22 may not need to use theentire area of each neighboring cell to rotate. As a result, spacereservation module 110 may allow other mobile drive units to placereservation requests on a particular neighboring cell at the same timethe first requesting mobile drive unit 20 and/or 22 has reserved thatparticular cell. More specifically, in particular embodiments, spacereservation module 110 may allow other mobile drive units 20 and/or 22to reserve neighboring cells for the purpose of encroaching into thatcell while rotating inventory holders 30 and/or 32 that border theneighboring cell.

Thus, although straight segments 17 may be utilized, particularembodiments of inventory management system 10 may be configured togenerate paths to include segments covering turns, curves or othernon-linear portions. For example, as illustrated, segment 17 e includesa curved portion. Additionally, although the illustrated example segment17 extends without limits, between turns and paths 16, particularembodiments of inventory management system 10 may be configured togenerate path 16 that has an upper limit on segment length or allows amaximum segment length to be reserved within a single reservation. Inaddition, mobile drive units 20 and/or 22 may be capable of requestingnew paths while still completing paths to request destinations. In thatway, mobile drive units 20 may store multiple tasks to complete, whichmay include multiple paths and/or multiple destinations. It should beunderstood however, that while the example embodiments discussed aboveillustrate examples of inventory management system 10 in which mobiledrive units 20 and/or 22 actively request reservations of particularsegments 17 on their own behalf, in alternative embodiments, spacereservation module 110 or other suitable components of inventorymanagement system 10 may be responsible for initiating reservations. Forexample, management module 15 may monitor the location and current pathof mobile drive units 20 and/or 22 and may reserve appropriate segments17 on behalf of mobile drive units 20 and/or 22 at appropriate timesduring the movement of mobile drive units 20.

Consequently, inventory management system 10 supports a number oftechniques that provide for efficient routing, navigation, andmanagement of mobile drive units 20 and/or 22 moving within workspace74. Inventory management system 10 supports techniques for resolvingand/or avoiding conflicting requests for a particular segment 17 by twodifferent mobile drive units 20 and/or 22. As a result, mobile driveunits 20 and/or 22 may operate within the same workspace 74 to providefor a more efficient use of system resources and inventory managementsystem 10.

FIGS. 6A and 6B are flowcharts illustrating an example embodiment of amethod 600 for inventory management. At step 602, a workspace 74 forfirst and second mobile drive units is provisioned. For example,multiple inventory holders of various sizes may be distributed intoworkspace 74, such as inventory holders 30 and/or 32. Various inventorystations 56 may be assigned in which mobile drive units 20 and/or 22interact. In some embodiments, the provisioning process may includedefining a set of workspace volumes 75 describing variousthree-dimensional characteristics of workspace 74. In addition or in thealternative, the provisioning process may include provisioning a set ofworkspace cells 76 describing two-dimensional characteristics ofworkspace 74, such as a map or floor layout.

At step 604, spaces representing the first and second mobile drive unitsare defined. For example, a three-dimensional volume segment may bedefined for mobile drive unit 20 and a second three-dimensional volumesegment may be defined for mobile drive unit 22. Each of the volumes isrespectively associated with the physical dimensions of mobile driveunit 20 and 22. As another example, footprints representing thetwo-dimensional projections of the first mobile drive unit and secondmobile drive units onto a surface of workspace 74 may be created.

At step 606, the first mobile drive unit 20 is deployed into inventorymanagement system 10. Mobile drive unit 20, for example, may registerits location and/or assignment status with management module 15.

At step 608, resource scheduling module 106 determines if first mobiledrive unit 20 is currently completing a task. Resource scheduling module106 may have a task that involves a given inventory holder 30. If firstmobile drive unit 20 is currently completing another task, resourcescheduling module 106 may determine whether another mobile drive unit 20is available to complete that task or may wait until the first mobiledrive unit 20 is available. If first mobile drive unit 20 is notcurrently completing a task, resource scheduling module 106 may, at step610, assign a task to the first mobile drive unit 20.

First mobile drive unit 20 may request a path from management module 15and, at step 612, route planning module 108 determines a path 16 forfirst mobile drive unit 20. According to the techniques discussed above,once one or more paths 16 are determined by route planning module 108and are transmitted to first mobile drive unit 20.

Once first mobile drive unit 20 receives the one or more paths 16, atstep 614, first mobile drive unit 20 communicates a reservation requestto space reservation module 110. For example, first mobile drive unit 20may request a volume segment associated with the physical dimensions offirst mobile drive unit 20. If first mobile drive unit 20 is coupledand/or docked with an inventory holder 30, space reservation module 110modifies the volume segment associated with first mobile drive unit 20based on the physical dimensions of inventory holder 30. As anotherexample, first mobile drive unit 20 may request a segment representingthe footprint of first mobile drive unit 20 in two-dimensional space.

At step 616, space reservation module 110 may determine whether thereservation request conflicts with a reservation policy. For example,reservation policies may include any of the policies discussed above. Ifthe requested segment does not conflict with a reservation policy, themethod may determine at step 618 whether the requested segmentintersects with a reserved space. If either the requested reservationconflicts with a reservation policy or the requested segment has alreadybeen reserved by another component, then the method may return to step612 and route planning module 108 determines a new path 16 for firstmobile drive unit 20. In some embodiments, however, space reservationmodule 110 may first determine whether a lesser portion of the requestedsegment may be granted. For example, a lesser segment may be granted ifthe segment allows for some movement of the mobile drive unit 20 in thedesired direction. If a lesser portion cannot be granted or therequested segment intersects with a permanent reservation, the methodmay then return to step 612. If, however, the requested segmentintersects with a temporary reservation, space reservation module 110may wait a predetermined amount of time and attempt to reserve thesegment again. Space reservation module 110 may retry a number of timesbefore determining a new path. Additionally or alternatively, spacereservation module 110 may request a segment of a second alternativepath based on priority.

At step 620, if the requested segment does not intersect with a reservedspace and the reservation request does not conflict with a reservationpolicy, then the space is reserved for first mobile drive unit 20. Firstmobile drive unit 20 will then move along the reserved segment.

At step 622, if the destination has not been reached, then the methodreturns to step 614 where a second segment of the path 16 is requestedand the method repeats. In some embodiments, first mobile drive unit 20may request the next segment of path 16 during traversal of the reservedsegment, and in some embodiments, may release reservations of the spaceas first mobile drive unit 20 traverses through it.

Once the destination is reached, then the task is completed at step 624.For example, the task may include docking with an inventory holder 30 ata destination. Once mobile drive unit 20 reaches that destination, itmay dock with inventory holder 30 and transmit information to managementmodule 15 indicating the assigned task is complete. Resource schedulingmodule 106 may then assign a next task to first mobile drive unit 20and/or first mobile drive unit 20 may attempt to begin a next taskstored in first mobile drive unit 20.

Likewise, at step 626, a second mobile drive unit 22 is deployed in asimilar manner to first mobile drive unit 20. The second mobile driveunit 22 has a physical dimension different from the first mobile driveunit 20 and is capable of transporting inventory holders 32 havingdifferent dimensions than the inventory holders 30 mobile drive unit 20is capable of transporting in the same workspace.

At step 628, resource scheduling module 106 determines if second mobiledrive unit 22 is currently completing a task. Resource scheduling module106 may have a task that involves a given inventory holder 32. If secondmobile drive unit 22 is currently completing another task, resourcescheduling module 106 may determine whether another mobile drive unit 22is available to complete that task or may wait until the second mobiledrive unit 22 is available. If second mobile drive unit 22 is notcurrently completing a task, resource scheduling module 106 may, at step630, assign a task to second mobile drive unit 22.

Second mobile drive unit 22 may request a path 16 from management module15 and, at step 632, route planning module 108 determines a path 16 forthe second mobile drive unit 22. In some embodiments, path 16 plannedfor the second mobile drive unit 22 may depend on the other path 16planned for the first mobile drive unit 20. Additionally oralternatively, the set of reservations in workspace 74 may change fromthe time the first path 16 is planned for the first mobile drive unit 20to the time the second path 16 is planned for the second mobile driveunit 22. The planned path 16 may change accordingly. The available paths16 may also vary according to the physical dimensions of mobile driveunit 22, which may prevent the second mobile drive unit 22 from takingthe same routes as a given mobile drive unit 20. According to thetechniques discussed above, one or more paths 16 are determined by routeplanning module 108 and are transmitted to second mobile drive unit 22.

Once second mobile drive unit 22 receives the one or more paths 16, atstep 634, second mobile drive unit 22 communicates a reservation requestto management module 15. For example, second mobile drive unit 22 mayrequest a volume segment associated with the physical dimensions ofsecond mobile drive unit 22. If second mobile drive unit 22 is coupledand/or docked with an inventory holder 32, management module 15 modifiesthe volume segment associated with second mobile drive unit 22 based onthe physical dimensions of inventory holder 32. As another example,second mobile drive unit 22 may request a segment corresponding to thefootprint of mobile drive unit 22 in two-dimensional space.

At step 636, space reservation module 110 may determine whether thereservation request conflicts with a reservation policy. For example,reservation policies may include any of the policies discussed above. Ifthe requested segment does not conflict with a reservation policy, themethod may determine at step 638 whether the requested segmentintersects with a reserved space. If either the requested reservationconflicts with a reservation policy or the requested segment has alreadybeen reserved, then the method may return to step 632 and spacereservation module 110 determines a new path for second mobile driveunit 22. In some embodiments, however, space reservation module 110 mayfirst determine whether a lesser portion of the requested segment may begranted. For example, a lesser segment may be granted if the segmentallows for some movement of the mobile drive unit 22 in the desireddirection. If a lesser portion cannot be granted or the requestedsegment intersects with a permanent reservation, the method may thenreturn to step 632. If, however, the requested segment intersects with atemporary reservation, space reservation module 110 may wait apredetermined amount of time and attempt to reserve the segment again.Space reservation module 110 may retry a number of times beforedetermining a new path. Additionally or alternatively, space reservationmodule 110 may request a segment of a second alternative path based onpriority.

At step 640, if the requested segment does not intersect with a reservedspace and the reservation request does not conflict with a reservationpolicy, then the space is reserved for the second mobile drive unit 22.Second mobile drive unit 22 will then move along the reserved segment.

At step 642, if the destination has not been reached, then the methodreturns to step 634 where a second segment of the path is requested andthe method repeats. In some embodiments, second mobile drive unit 22 mayrequest the next segment of the path during traversal of the reservedsegment, and in some embodiments, may release reservations of the spaceas second mobile drive unit 22 traverses through it.

Once the destination is reached, then the task is completed at step 644.For example, the task may be to dock with an inventory holder 32 at adestination. Once second mobile drive unit 22 reaches that destination,it may dock with inventory 32 and transmit information to managementmodule 15 indicating the assigned task is complete. Resource schedulingmodule 106 may then assign a next task to second mobile drive unit 22and/or second mobile drive unit 22 may attempt to begin a next taskstored by second mobile drive unit 22.

It should be noted that while the method 600 illustrates the first andsecond mobile drive units as being deployed sequentially, the method isfully intended to allow various mobile drive units to operatesimultaneously and these steps are illustrated as happening sequentiallyfor a purpose of illustration only. In addition, modifications,additions, or omissions may be made to method 600 illustrated in theflowchart of FIG. 6. For example, method 600 may process severalrequests for reservations once and/or in parallel. Additionally, thesteps of FIG. 6 may be performed in parallel or in any suitable order.

Technical advantages of certain embodiments of the present inventioninclude the ability to deploy a first mobile drive unit having firstdimensions and a second mobile drive unit having second dimensions in aninventory management system. In certain embodiments, the first andsecond mobile drive units may operate within the same workspace. In someembodiments, the first and second mobile drive units may haveoverlapping or distinct workspaces. Another technical advantage mayinclude transporting inventory holders that have different physicalsizes and/or purposes. Accordingly, in some embodiments different-sizedmobile drive units may each serve the same inventory station. Accordingto the teachings of the present disclosure, the space and time requiredto accomplish the same task as compared to a system that does not deploydifferent-sized mobile drive units may be substantially reduced andcertain operational efficiencies realized.

Although the present invention has been described with severalembodiments, a myriad of changes, variations, alterations,transformations, and modifications may be suggested to one skilled inthe art, and it is intended that the present invention encompass suchchanges, variations, alterations, transformations, and modifications asfall within the scope of the appended claims.

What is claimed is:
 1. A method for inventory management, comprising:deploying a first mobile drive unit having first dimensions; deploying asecond mobile drive unit having second dimensions, wherein the first andsecond dimensions are different; and wherein the first and second mobiledrive units are operable to transport inventory items to a plurality ofinventory stations in the same workspace.
 2. The method of claim 1,further comprising determining a first path for the first mobile driveunit; determining a second path for the second mobile drive unit; andreserving a first segment of the first path, the first segmentcomprising a first space associated with the first dimensions of thefirst mobile drive unit.
 3. The method of claim 2, wherein the firstspace comprises one of a volume, a cell, and a footprint.
 4. The methodof claim 2, further comprising: receiving a request to reserve a secondsegment of a second path; determining whether the requested secondsegment intersects with a reserved space; and determining that therequested second segment does not intersect with the reserved space. 5.The method of claim 2, wherein reserving a first segment furthercomprises: determining that the first mobile drive unit is coupled to aninventory holder; modifying the first space based on the dimensions ofthe inventory holder; and reserving a modified first segment comprisingthe modified first space.
 6. The method of claim 2, further comprising:applying a reservation policy, wherein reserving the first segment isbased on determining that the reservation does not conflict with thereservation policy.
 7. The method of claim 6, wherein the reservationpolicy is based on at least one of an inventory holder location, apredetermined number of maximum drives, a predetermined maximum weight,a predetermined maximum speed, and an inventory type.
 8. The method ofclaim 2, wherein reserving a first segment further comprises: receivinga request to reserve the first segment of the first path, the firstsegment representing a physical location of an inventory holder;determining that the first segment does not intersect with the inventoryholder; and after reserving the first segment, moving the first mobiledrive unit under the inventory holder.
 9. The method of claim 2, whereinreserving a first segment further comprises: receiving a request toreserve the first segment of the first path; determining that therequested first segment comprises a partially unreserved portion; andreserving the partially unreserved portion.
 10. The method of claim 1,further comprising: determining a capability of one of the plurality ofinventory stations; and assigning an order associated with the first andsecond mobile drive units to the inventory station based on thecapability.
 11. The method of claim 2, wherein reserving a first segmentfurther comprises: receiving a request to reserve the first segment ofthe first path; determining that the requested first segment comprises apartially reserved portion and a partially unreserved portion; partiallygranting the request of the partially unreserved portion; furtherdetermining that an additional portion of the partially reserved portionhas become unreserved; and partially granting the request of theadditional portion.
 12. The method of claim 11, further comprising:determining that the additional portion conflicts with a reservationpolicy; and reducing the additional portion based on the reservationpolicy.
 13. The method of claim 1, wherein the first and second mobiledrive units are each operable to transport inventory items to one of theplurality of inventory stations in the same workspace.
 14. A system forinventory management comprising: a first mobile drive unit having firstdimensions; a second mobile drive unit having second dimensions, whereinthe first and second dimensions are different; and a plurality ofinventory holders, the first and second mobile drive units each operableto transport at least one of the inventory holders.
 15. The system ofclaim 14, further comprising a management module operable to: determinea first path for the first mobile drive unit; determine a second pathfor the second mobile drive unit; and reserve a first segment of thefirst path, the first segment comprising a first space associated withthe first dimensions of the first mobile drive unit.
 16. The system ofclaim 15, wherein the first space comprises one of a volume, a cell, anda footprint.
 17. The system of claim 15, the management module furtheroperable to: receive a request to reserve a second segment of a secondpath; determine whether the requested second segment intersects with areserved space; and determine that the requested second segment does notintersect with the reserved space.
 18. The system of claim 15, themanagement module further operable to: determine that the first mobiledrive unit is coupled to an inventory holder; modify the first spacebased on the dimensions of the inventory holder; and reserve a modifiedfirst segment comprising the modified first space.
 19. The system ofclaim 15, the management module further operable to: apply a reservationpolicy, wherein reserving the first segment is based on determining thatthe reservation does not conflict with the reservation policy.
 20. Thesystem of claim 19, wherein the reservation policy is based on at leastone of an inventory holder location, a predetermined number of maximumdrives, a predetermined maximum weight, a predetermined maximum speed,and an inventory type.
 21. The system of claim 15, the management modulefurther operable to: receive a request to reserve the first segment ofthe first path, the first segment representing a physical location of aninventory holder; determine that the first segment does not intersectwith the inventory holder; and wherein after reserving the firstsegment, the first mobile drive unit moves under the inventory holder.22. The system of claim 15, the management module further operable to:receive a request to reserve the first segment of the first path;determine that the requested first segment comprises a partiallyunreserved portion; and reserve the partially unreserved portion. 23.The system of claim 14, further comprising a management module operableto: determine a capability of one of a plurality of inventory stations;and assign an order associated with the first and second mobile driveunits to the inventory station based on the capability.
 24. The systemof claim 15, the management module further operable to: receive arequest to reserve the first segment of the first path; determine thatthe requested first segment comprises a partially reserved portion and apartially unreserved portion; partially grant the request of thepartially unreserved portion; further determine that an additionalportion of the partially reserved portion has become unreserved; andpartially grant the request of the additional portion.
 25. The system ofclaim 24, the management module further operable to: determine that theadditional portion conflicts with a reservation policy; and reduce theadditional portion based on the reservation policy.
 26. The system ofclaim 14, further comprising a plurality of inventory stations, whereinthe first and second mobile drive units are each operable to transport aleast one of the plurality of inventory holders to a least one of theplurality of inventory stations.
 27. The system of claim 26, wherein theplurality of inventory stations comprise: a plurality of first inventorystations operable to interact with inventory items from the first mobiledrive units; a plurality of second inventory stations operable tointeract with inventory items from the second mobile drive units; and aplurality of third inventory stations operable to interact withinventory items from the first and second mobile drive units.
 28. Thesystem of claim 14, wherein the plurality of inventory holders comprise:a plurality of first inventory holders having third dimensions; aplurality of second inventory holders having fourth dimensions, whereinthe third and fourth dimensions are different; the first mobile driveunit is further operable to transport the plurality of first inventoryholders; and the second mobile drive unit is further operable totransport the plurality of second inventory holders.
 29. The system ofclaim 28, wherein the first mobile drive unit is further operable totransport the second inventory holders.
 30. The system of claim 28,wherein the second mobile drive unit is further operable to transportthe first inventory holders.
 31. A management module operable to:facilitate management of a first mobile drive unit having firstdimensions in an inventory management system; facilitate management of asecond mobile drive unit having second dimensions in the inventorymanagement system, wherein the first and second dimensions aredifferent; and wherein the first and second mobile drive units areoperable to transport inventory items to a plurality of inventorystations in the inventory management system.
 32. The management moduleof claim 31, further operable to: determine a first path for the firstmobile drive unit; determine a second path for the second mobile driveunit; and reserve a first segment of the first path, the first segmentcomprising a first space associated with the first dimensions of thefirst mobile drive unit.
 33. The management module of claim 32, whereinthe first space comprises one of a volume, a cell, and a footprint. 34.The management module of claim 32, further operable to: receive arequest to reserve a second segment of a second path; determine whetherthe requested second segment intersects with a reserved space; anddetermine that the requested second segment does not intersect with thereserved space.
 35. The management module of claim 32, further operableto: determine that the first mobile drive unit is coupled to aninventory holder; modify the first space based on the dimensions of theinventory holder; and reserve a modified first segment comprising themodified first space.
 36. The management module of claim 32, furtheroperable to: apply a reservation policy, wherein reserving the firstsegment is based on determining that the reservation does not conflictwith the reservation policy.
 37. The management module of claim 36,wherein the reservation policy is based on at least one of an inventoryholder location, a predetermined number of maximum drives, apredetermined maximum weight, a predetermined maximum speed, and aninventory type.
 38. The management module of claim 32, further operableto: receive a request to reserve the first segment of the first path,the first segment representing a physical location of an inventoryholder; determine that the first segment does not intersect with theinventory holder; and wherein after reserving the first segment, thefirst mobile drive unit moves under the inventory holder.
 39. Themanagement module of claim 32, further operable to: receive a request toreserve the first segment of the first path; determine that therequested first segment comprises a partially reserved portion and apartially unreserved portion; and partially grant the request of thepartially unreserved portion.
 40. The management module of claim 31,further operable to: determine a capability of one of the plurality ofinventory stations; and assign an order associated with the first andsecond mobile drive units to the inventory station based on thecapability.
 41. The management module of claim 32, further operable to:receive a request to reserve the first segment of the first path;determine that the requested first segment comprises a partiallyreserved portion and a partially unreserved portion; partially grant therequest of the partially unreserved portion; further determine that anadditional portion of the partially reserved portion has becomeunreserved; and partially grant the request of the additional portion.42. The management module of claim 41, further operable to: determinethat the additional portion conflicts with a reservation policy; andreduce the additional portion based on the reservation policy.